This invention is directed to a mixhead for use in a reaction injection molding (RIM) machine in which the molding resin is composed of two or more liquid components which are mixed within the mixhead and then injected into a mold where they react to a high polymer. One such machine is shown in Macosko et al U.S. Pat. No. 4,189,070, issued Feb. 19, 1980. The mixhead includes a mixing chamber with two or more inlets for the separate liquid resin components which are introduced under pressure from vessels containing the separate liquids. The mixhead operates intermittently. The mixed components are dispensed from the mixing chamber by high pressure flow of the components which is interrupted by action of a reciprocal piston or ram. Just before mixing it is desirable that the components be recirculated back to the supply vessel. More particularly, this invention is directed to a new impingement type RIM mixhead with high pressure recycle.
One of the most important problems in designing impingement mixheads for reaction injection molding is the start-up transient. This is known as "lead-lag" in the industry since one component stream leads or lags the other in entering the chamber. Another transient problem can be low initial flow rate or Reynolds number, and thus poor mixing. Because in RIM the mixhead is used intermittently and for a short period of from about 1 to 3 seconds, the transient can be a significant part of the cycle. Even a small amount of lead-lag can cause streaks and weak sections in finished parts.